Types A and B Niemann-Pick disease (NPD) are lysosomal storage disorders (LSDs) resulting from the deficient activity of acid sphingomyelinase (ASM), and the subsequent accumulation of sphingomyelin, cholesterol, and other lipids within cells and tissues of affected individuals (Schuchman and Desnick, Niemann-Pick disease types A and B: acid sphingomyelinase deficiencies. In: The metabolic and molecular bases of inherited disease. Edited by Schriver C R, Beaudet A L, Valle D, Sly W S; New York. McGraw Hill Inc 3589-3610 2001). Patients with Type A NPD are usually diagnosed early in infancy with organomegaly and follow a rapid, neurodegenerative course that leads to death by about three years of age. In contrast, patients with Type B NPD have little or no central nervous system involvement and often survive into adulthood. However, the Type B form of NPD is clinically heterogeneous and can present with a variety of findings that may include hepatosplenomegaly, growth retardation, frequent respiratory infections, fatigue, and hematologic abnormalities such as high LDL cholesterol and triglycerides, low HDL cholesterol, and low platelets. In addition, several Type B NPD patients have been reported with an intermediate phenotype that involved neurodegeneration (Elleder and Cihula, Eur J Pediatr 1983; 140:323-328; Elleder et al., J Inherit Metab Dis 1986; 9:357-366).
Both forms of NPD are panethnic, although most reported Type A NPD cases occurred among Ashkenazi Jewish individuals. Type B NPD, by contrast, appears to be more prevalent in North African, Arab, and Turkish populations. To date, over 70 mutations in the ASM gene have been reported causing Types A or B NPD (Simonaro et al., Am J Hum Gen. 2002, 71:1413-1419). Among these are a small number of common mutations that predict specific phenotypes. For example, the delta R608 mutation, wherein an arginine at residue 608 (AR608) is deleted, is found in about 10-15% of all NPD patients in North American and Western Europe, and is always associated with a non-neurological form of the disease (i.e., Type B NPD) (Levran et al., J Clin Invest. 1991; 88:806-810). This mutation also is found in about 90% of Type B NPD patients from North Africa (Vanier et al., Hum Genet. 1993; 92:325-330). In contrast to the AR608 mutation, three additional mutations, L302P, where proline replaces leucine at amino acid residue 302, R496L, where leucine replaces arginine at amino acid residue 496, and fsP330, where a premature stop codon is introduced downstream of a proline at amino acid residue 330, account for more than 90% of Type A NPD patients in the Ashkenazi Jewish population (Levran et al., Proc Natl Acad Sci USA. 1991; 88:3748-3752; Levran et al., Blood. 1992; 80:2081-2087; Levran et al., Hum Mut. 1993; 2:317-319). The carrier frequency for these three mutations within the Ashkenazi Jewish community is about 1:80 to 1:100 (Li et al. Am J Hum Genet. 1997; 61 (suppl):A24).
Recently, three isoforms of the human ASM gene were cloned and several mutations were identified that can reliably be used in diagnostic evaluations of obligate heterozygotes for NPD Types A and B in the Ashkenazi Jewish population (see U.S. Pat. Nos. 5,773,278 and 5,686,240 to Schuchman et al.).
Supportive management is the only treatment available for most LSD patients. Enzyme replacement therapy (ERT) has been developed or is currently under development for several LSDs, including Gaucher disease, Fabry disease, and mucopolysaccharidosis Type I (MPS I) (Desnick and Schuchman, Nat Rev Genet. 2002; 3:954-966), but since the enzymes do not cross the blood brain barrier after intravenous infusion, this is not a useful strategy for patients with severe neurological involvement (e.g., Type A NPD). Substrate deprivation therapy, which uses small molecule inhibitors to prevent the synthesis of pathogenic substrates, also has been developed or is under evaluation for several LSDs, and has conditional marketing approval in Europe and the United States for Gaucher disease (Butters et al., Philos Trans R Soc Lond B Biol Sci. 2003; 358:927-945.). One advantage of this approach as compared to ERT is that the small molecule inhibitors may potentially cross the blood brain barrier and prevent substrate accumulation in the brain.
Another small molecule approach recently developed is known as chemical chaperone, or active site-specific chaperone (ASSC) therapy (Fan et al Nat Med. 1999; 5: 112-115; Fan, Trends Pharmacol Sci. 2003; 24: 355-360). ASSC uses low concentrations of potent enzyme inhibitors to enhance the folding and activity of mutant proteins in specific LSDs. This approach was first evaluated in Fabry disease, where a small molecule inhibitor of alpha-galactosidase A, 1-deoxy-galactononjirimycin (DGJ), was used to enhance the residual alpha-galactosidase activity in cell lines from Fabry disease patients (see U.S. Pat. No. 6,274,597 to Fan et al.). U.S. Pat. No. 6,583,158, to Fan et al. further exemplifies the ASSC strategy for numerous other lysosomal storage diseases, including Gaucher disease and GM1-gangliosidosis, and demonstrates that this therapeutic strategy of using potent competitive inhibitors as chemical chaperones to enhance the residual enzyme activity in a patient's cells is not limited to patients with Fabry disease, but can be applied to numerous enzyme deficiency diseases, particularly LSDs.
ASSC therapy is now currently under development for several LSDs, including Gaucher disease, and offers several advantages over ERT or substrate deprivation therapy. Most notably, since the active site inhibitors used in ASSC are specific for the disease-causing enzyme, the therapy is targeted to a single protein and metabolic pathway, unlike substrate deprivation therapy that inhibits an entire synthetic pathway. Like substrate deprivation therapy, the small molecule inhibitors for ASSC have the potential of crossing the blood brain barrier and could be used to treat neurological LSD forms.
In addition to enhancing the activity of the deficient enzymes associated with the LSDs, the ASSCs have also been demonstrated to enhance the activity of the corresponding wild-type enzyme (see U.S. Pat. No. 6,589,964 to Fan et al.), thus having utility as co-therapy for enzyme replacement therapy in LSD patients.
Type A NPD is an important candidate for ASSC. First, all patients with Type A NPD develop a severe, neurological phenotype that leads to death by about three years of age. There is currently no treatment for the neurological features of this disorder. However, studies using an ASM knock-out mouse model have shown that enhancing the residual ASM activity up to about 5% of normal activity can completely prevent the occurrence of brain disease and lead to a normal lifespan, suggesting that a low level of functional ASM prevents neurodegeneration in Type A NPD patients (Marthe et al., Hum Mol Genet. 2000; 9: 1967-1976). In addition, two of these three mutations (L302P and R496L) that are responsible for most Ashkenazi Jewish Type A NPD patients are point mutations that might be amenable to ASSC. In addition, about 15% of Type B NPD patients worldwide carry at least one copy of the ΔR608 mutation. Thus, there is a substantial need for this form of small molecule therapy.